When a charge of solid copper metal is heated and melted in a vertical shaft melting furnace, the charge melts from the bottom of the stack. The charge of metal within the furnace settles downwardly within the shaft and material is added to the top of the charge contained within the shaft. This process is continuous for as long as molten metal is needed. Slags form which are a natural result of this melting process. As the copper melts, the molten metal falls down onto the hearth of the shaft furnace, pools, and flows toward a low point in the hearth where a taphole or exit port is located. Prior art shaft furnaces provided air and gas fired burners which, when used to fire such a furnace, produced a metal on the hearth at only a slightly higher temperature than the melting point of the metal. This relatively cool metal, along with slag that forms in the chamber, have a tendency to freeze on the hearth or in the taphole. Slags, such as those described in the above system, have higher melting points than does the copper and are therefore more likely to freeze at or around the exit port. In the typical system, when this cool metal, and slags which have a relatively higher melting point, reach a cooler portion of the furnace, such as the taphole, they do not flow rapidly enough to keep the taphole free and flowing, and will often freeze up, thereby closing the taphole or exit port. When this frozen condition exists, the system must be shut down while a source of heat, such as an oxygen lance, is used to remelt the frozen mass and thereby open the taphole and restart the flow of metal from the furnace.
This situation is common place whether the burners used to fire the furnace are of a liquid fuel type, a gas and air postmix type, or an air and gas premix type of burner. These typical burners are only capable of melting copper from the bottom of the charge stack with the molten metal produced being only slightly above its melting temperature. An increase in the firing rate of these burners simply results in a greater flow of metal being melted from the bottom of the charge. However, the increased flow comprises metal still having the same relatively low temperature as that produced at the lower firing rate.
It is very common place for a shaft furnace to comprise a plurality of burners employed to heat and melt the charge of copper metal within the shaft furnace. The problem of the slag, causing the melt to refreeze before it can exit the furnace, is of major concern. In the manufacture of copper products, processes which prevent down times are essential. The melting and casting of copper metal cannot be interrupted to restart a frozen furnace if the process is to be efficiently carried out. When time must be taken from production of a product, in order to open a clogged furnace, production time and the related production expenses both go up.
It is therefore desirable that a method be devised whereby the molten copper and the related slags could be elevated to such a temperature that they will remain fluid, and easily manageable and flow freely from the hearth of the furnace thereby eliminating the down time necessary to remelt and allow these materials to flow from the system. It is to this end that the implementation of a burner capable of being fed gas, air, and oxygen comes into its own right. A burner being fed a controllable mixture of all three of these gases is typically designed such that its burning efficiency and temperature are precisely controllable. It is possible to achieve higher flame temperatures with such a burner than can be obtained with a burner combusting only air and fuel. U.S. Pat. No. 4,586,895 disclosed such a burner. The stated purpose of the design of this referenced burner is the elimination of slag from the mouth of the burner itself as the burner is used in applications of melting steel. After a given period of use, the oxygen/gas/air mixture is unbalanced such that an oxygen rich flame is produced. The oxygen rich portion of this flame, the oxygen being heated and applied at relatively high temperatures, results in the post oxidization and elevated temperatures of the slag accumulation at the mouth of the burner. The '895 patent teaches this oxidization of the slag, and the resulting high temperatures, causing the slag to become fluid so as to be easily removed from the burner throat by the natural action of the gases passing through said burner and flushing the slag away.
Another feature of the typical vertical shaft melting furnace deals with the quality of the metal used to charge said furnace. The composition of the metal which exits the shaft furnace is, with very small exception, identical to the composition of the metal used to charge the furnace. That is to say, there is no refining action associated with the use of a typical shaft furnace when used to melt copper. This feature of the vertical shaft melting furnace requires that a relatively high grade of feed material be used in the charge. The use of a higher grade material results in a correspondingly higher cost of operating the furnace. This is due to the cost of higher grade material being greater than the cost of lower grade material.
Many of the above enumerated draw backs can be overcome through the use of a gas, air, and oxygen burner.